<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        console.log(a);//未定义
        var a;
        const b = 2;
        const c = 456;
        c = 456;
        const d = {
            name:'tom'
        }
        d.age = 18;
        console.log(d);
    </script>
    <h1>1</h1>
</body>
</html>